Cytoplasmic protein complex comprising a kinase substrate sensor cells and associated detection methods

ABSTRACT

The disclosure relates to a cytoplasmic protein complex comprising: (a) a first recombinant fusion protein comprising a kinase, fused to a first interaction polypeptide; and (b) a second recombinant fusion protein comprising a domain comprising a reporter phosphorylation site, whereby the domain is fused to a second interaction polypeptide. The disclosure relates further to a method to detect compound-compound-interaction using the cytoplasmic protein complex, and to cells comprising such cytoplasmic protein complex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/EP2012/053463, filed Feb. 29, 2012, designating the United States of America and published in English as International Patent Publication WO 2012/117031 A1 on Sep. 7, 2012, which claims the benefit under Article 8 of the Patent Cooperation Treaty and under 35 U.S.C. § 119(e) to Great Britain Patent Application Serial No. 1103453.5, filed Mar. 1, 2011, and to U.S. Provisional Patent Application Ser. No. 61/464,285, filed Mar. 1, 2011.

TECHNICAL FIELD

The disclosure relates to a cytoplasmic protein complex comprising (a) a first recombinant fusion protein comprising a kinase, fused to a first interaction polypeptide and (b) a second recombinant fusion protein comprising a domain comprising a reporter phosphorylation site, wherein the domain is fused to a second interaction polypeptide. It relates further to a method of detecting compound-compound-interaction using the cytoplasmic protein complex, and to cells comprising such cytoplasmic protein complex.

BACKGROUND

Protein-protein interactions are an essential key in all biological processes, from the replication and expression of genes to the morphogenesis of organisms. Protein-protein interactions govern, amongst others, ligand-receptor interaction and the subsequent signaling pathway; they are important in assembly of enzyme subunits, in the formation of biological supramolecular structures such as ribosomes, filaments and virus particles, and in antigen-antibody interactions.

Researchers have developed several approaches in attempts to identify protein-protein interactions. Co-purification of proteins and co-immunoprecipitation were amongst the first techniques used. However, these methods are tedious and do not allow high throughput screening. Moreover, they require lysis corrupting the normal cellular context. A major breakthrough was obtained by the introduction of the genetic approaches, of which the yeast two-hybrid (Fields and Song, 1989) is the most important one. Although this technique became widely used, it has several drawbacks. The fusion proteins need to be translocated to the nucleus, which is not always evident. Proteins with intrinsic transcription activation properties may cause false positives. Moreover, interactions that are dependent upon secondary modifications of the protein such as phosphorylation cannot be easily detected.

Several alternative systems have been developed to solve one or more of these problems.

Approaches based on phage display do avoid the nuclear translocation. WO9002809 describes how a binding protein can be displayed on the surface of a genetic package, such as a filamentous phage, wherein the gene encoding the binding protein is packaged inside the phage. Phages, which bear the binding protein that recognizes the target molecule, are isolated and amplified. Several improvements of the phage display approach have been proposed, as described, e.g., in WO9220791, WO9710330 and WO9732017.

However, all these methods suffer from the difficulties that are inherent at the phage display methodology: the proteins need to be exposed at the phage surface and are so exposed to an environment that is not physiologically relevant for the in vivo interaction. Moreover, when screening a phage library, there will be a competition between the phages that results in a selection of the high affinity binders.

U.S. Pat. No. 5,637,463 describes an improvement of the yeast two-hybrid system, whereby a phage library can be screened for modification-dependent protein-protein interactions. However, this method relies on the co-expression of the modifying enzyme, which will exert its activity in the cytoplasm and may modify enzymes other than the one involved in the protein-protein interaction, which may, on its turn, affect the viability of the host organism.

An interesting evolution is described in U.S. Pat. No. 5,776,689, by the so-called protein recruitment system. Protein-protein interactions are detected by recruitment of a guanine nucleotide exchange factor (Sos) to the plasma membrane, where Sos activates a Ras reporter molecule. This results in the survival of the cell that otherwise would not survive in the culture conditions used. Although this method certainly has the advantage that the protein-protein interaction takes place under physiological conditions in the submembranary space, it has several drawbacks. Modification-dependent interactions cannot be detected. Moreover, the method is using the pleiotropic Ras pathway, which may cause technical complications.

A major improvement in the detection of protein-protein interactions was disclosed in WO0190188, describing the so called Mappit system. The method, based on a cytokine receptor, allows not only a reliable detection of protein-protein interactions in mammalian cells, but also modification-dependent protein interactions can be detected, as well as complex three-hybrid protein-protein interactions mediated by a small compound (Caligiuri et al., 2006). However, although very useful, the system is limited in sensitivity and some weak interactions cannot be detected. Moreover, as this is a membrane-based system, nuclear interactions are normally not detected.

There is still a need for a sensitive identification system for compound-compound interactions that can study these interactions under physiological conditions, with a low background and by which modification-dependent protein-protein interactions can be isolated.

DISCLOSURE

The disclosure, based on a mutant cytoplasmic kinase, preferably a constitutive mutant cytoplasmic kinase, satisfies this need and provides additional advantages as well. Whereas Mappit is using a ligand-receptor-controlled inducible kinase to limit and correct for the background, the cytoplasmic kinase, especially the constitutive cytoplasmic kinase, is surprisingly giving a higher signal/noise ratio, allowing the detection of interactions that cannot be detected in the mappit system. The high signal/noise ratio is unexpected for a cytoplasmic system, as a person skilled in the art would expect that cytoplasmic kinase can move freely in the cytoplasm, and reach its substrate without other recruitment signals than the kinase and the phosphorylation domain. By using a cytoplasmic system, the problem of detecting nuclear protein-protein interactions has been solved. The unexpectedly high sensitivity of the present method allows detecting weak protein interactions, such as the p51/p51 interaction, which cannot be detected by Mappit. Alternatively, a mutant kinase inducible by an exogenous small compound can be used. This approach combines the advantages of the constitutive kinase, with the inducibility, without interfering with the endogenous pathways.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1: Schematic representation of the cytoplasmic protein complex. A first interacting polypeptide (X) is fused to a constitutive kinase (K) and a second interacting polypeptide (Y) is fused to a reporter phosphorylation site (R). Interaction between the interacting polypeptides X and Y results in the reporter phosphorylation site being phosphorylated (P) by the constitutive kinase, leading to a detectable activity.

FIGS. 2A and 2B: Detection of the interaction between HIV1 Reverse Transcriptase (RT) subunits in an assay variant that comprises mutant Tyk2 kinase fusion proteins. FIG. 2A: Schematic overview of the assay. The first interacting polypeptide (X) is fused to the C-terminal region of Tyk2 comprising the kinase domain, whereas the second interacting polypeptide (Y) is fused to a fragment of gp130, which contains phosphorylation sites. When polypeptides X and Y interact, the Tyk2 kinase domain phosphorylates the phosphorylation sites of gp130. STAT3 transcription factors are recruited to these phosphorylated sites and are, in turn, phosphorylated by the Tyk2 kinase domain, which results in their activation. Dimers of activated STAT3 transcription factors are able to bind the specific rPAPI promoter, which drives the expression of a firefly luciferase reporter gene. The activity of this reporter gene is measured as light production in a luciferase detection assay using a luminometer. FIG. 2B: Application to the analysis of HIV1 RT subunits. Cells were transfected with the following plasmids: a) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp51+pXP2d2-rPAPI-luciferase, b) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase, c) pMet7-HA-Tyk2(C)-RTp51+pMG2-RTp51+pXP2d2-rPAPI-luciferase. Luciferase activity is shown as fold induction relative to the luciferase activity measured in cells transfected with the same Tyk2(C) fusion, an unfused gp130 fragment and the luciferase reporter plasmid (pMet7-HA-Tyk2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in a) and b); pMet7-HA-Tyk2(C)-RTp51+pMG1+pXP2d2-rPAPI-luciferase in c). Error bars indicate standard deviation.

FIG. 3: Detection of the interaction between HIV1 Integrase (IN) and human lens epithelium-derived growth factor (“LEDGE”) and between human p53 and mouse double minute 2 homolog (“MDM2”). Cells were transfected with the following plasmids: a) pMet7-HA-Tyk2(C)-LEDGF+pMG2-IN+pXP2d2-rPAPI-luciferase and b) pMet7-HA-Tyk2(C)-MDM2+pMG1-p53+pXP2d2-rPAPI-luciferase. Luciferase activity is shown as fold induction relative to the luciferase activity measured in cells transfected with the same Tyk2(C) fusion, an unfused gp130 fragment and the luciferase reporter plasmid (pMet7-HA-Tyk2(C)-LEDGF+pMG1+pXP2d2-rPAPI-luciferase in a); pMet7-HA-Tyk2(C)-MDM2+pMG1+pXP2d2-rPAPI-luciferase in b)). Error bars indicate standard deviation.

FIG. 4: Dose-dependent disruption of the interaction between human p53 and MDM2 by Nutlin-3. Cells were transfected with the following plasmids: a) pMet7-HA-Tyk2(C)-p53(N)+pMG1+pXP2d2-rPAPI-luciferase; b) pMet7-HA-Tyk2(C)-p53(N)+pMG1-EFHA1+pXP2d2-rPAPI-luciferase; and c) pMet7-HA-Tyk2(C)-p53(N)+pMG2-MDM2+pXP2d2-rPAPI-luciferase. After transfection, cells were treated with 0-14-41-123-370-1111-3333 nM Nutlin-3, final concentration. Error bars indicate standard deviation.

FIG. 5: Detection of the interaction between HIV1 Reverse Transcriptase (RT) p66 subunits in assay variants that comprise mutant Jak2 or c-Src kinase fusion proteins. Cells were transfected with the following plasmids: a) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase; b) pMet7-HA-Jak2(C)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase; and c) pMet7-HA-c-Src(K)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase. Luciferase activity is shown as fold induction relative to the luciferase activity measured in cells transfected with the same RTp66-kinase fusion, an unfused gp130 fragment and the luciferase reporter plasmid (pMet7-HA-Tyk2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in a); pMet7-HA-Jak2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in b); pMet7-HA-c-Src(K)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in c)). Error bars indicate standard deviation.

FIG. 6: Detection of the interaction between HIV1 Reverse Transcriptase (RT) subunits in an inducible assay variant. Cells were transfected with the following plasmids: a) pMet7-HA-Tyk2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase; b) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp51+pXP2d2-rPAPI-luciferase; c) pMet7-HA-Tyk2(R1027A)-RTp66+pMG1+pXP2d2-rPAPI-luciferase; and d) pMet7-HA-Tyk2(R1027A)-RTp66+pMG2-RTp51+pXP2d2-rPAPI-luciferase. After transfection, cells were treated with 0-5-50 mM imidazole, final concentration. Error bars indicate standard deviation.

DETAILED DESCRIPTION

A first aspect hereof is a cytoplasmic protein complex comprising (a) a first recombinant fusion protein comprising a kinase fused to a first interaction polypeptide and (b) a second recombinant fusion protein comprising a domain comprising a reporter phosphorylation site, wherein the domain is fused to a second interaction polypeptide.

The first recombinant fusion protein need not comprise a reporter phosphorylation site. Preferably, the kinase is a mutant kinase, more preferably, a mutant tyrosine kinase. In one embodiment, the mutant tyrosine kinase is a constitutive kinase. In another embodiment, the mutant tyrosine kinase is an inactive mutant that is activated by addition of an exogenous small molecule. Such mutant kinase is known to the person skilled in the art, and has been described, as a non-limiting example, by Qiao et al. (2006) as a Src 388R/A mutant or a 391R/A mutation in the corresponding human Src protein (Accession number NP_938033, version NP_938033.1). Alternatively, it may be a similar mutation in the Jak kinase family, such as, but not limited to Tyk2 1027R/A.

In one embodiment, the kinase is a constitutive kinase mutant derived from Tyk2, such as, but not limited to, a constitutive Tyk2 deletion mutant and/or a Tyk2 V678F mutant. “Derived from Tyk2” as used herein means that the kinase is a part of the human Tyk2 non-receptor tyrosine-protein kinase (Genbank accession number NP_003322; version NP_003322.3; SEQ ID NO:26) or a mutant or variant thereof, wherein the part shows constitutive kinase activity. A variant, as a non-limiting example, is a homologue, paralogue or orthologue. “Homologues” of a protein encompass peptides, oligopeptides, polypeptides, proteins and enzymes having amino acid substitutions, deletions and/or insertions relative to the unmodified protein in question and having similar biological and functional activity as the unmodified protein from which they are derived. “Orthologues” and “paralogues” encompass evolutionary concepts used to describe the ancestral relationships of genes. “Paralogues” are genes within the same species that have originated through duplication of an ancestral gene; “orthologues” are genes from different organisms that have originated through speciation, and are also derived from a common ancestral gene. Preferably, the homologue, “orthologue” or “paralogue” has a sequence identity at protein level of at least 50%, 51%, 52%, 53%, 54% or 55%, 56%, 57%, 58%, 59%, preferably 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, more preferably 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, even more preferably 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, and most preferably 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more as measured in a BLASTp (Altschul et al., 1997; Altschul et al., 2005). Variants and parts thereof according to the invention do show kinase activity. Preferably, the part is a part with constitutive kinase activity, preferably fragment 589-1187 of SEQ ID NO:26. Alternatively, the part is the part corresponding to fragment 589-1187 of SEQ ID NO:26 in a homologue, paralogue or orthologue as defined above, wherein the part has constitutive kinase activity. In an alternative embodiment, the constitutive kinase is a constitutive kinase derived from a Jak kinase, preferably from a Jak kinase selected from the group consisting of Jak1 (Accession number P23458, version P23458.2), Jak2 (Accession number 060674, version 060674.2), and Jak3 (Accession number P52333, version P52333.2), or a mutant or variant thereof as defined above. Preferably, the constitutive kinase is a constitutive Jak2 deletion mutant. In still another alternative embodiment, the constitutive kinase is a constitutive kinase derived from a Src kinase (Accession number NP_005408, version NP_005408.1) or a mutant or variant thereof as defined above. Preferably, the constitutive kinase is a Src deletion mutant as depicted in SEQ ID NO:27. “Derived” as used herein means that the kinase is a part of the cited non-receptor tyrosine-protein, or from a mutant or variant thereof, wherein the part shows constitutive kinase activity.

In one embodiment, the first recombinant fusion protein comprises SEQ ID NO:1 (pseudo kinase+kinase). In another embodiment, the first recombinant protein comprises SEQ ID NO:2 (kinase). In still another embodiment, the first recombinant protein comprises a sequence selected from the group consisting of SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, and SEQ ID NO:27. In one embodiment, the second recombinant fusion protein comprises SEQ ID NO:3 (STAT site). In another embodiment, the second recombinant fusion protein comprises SEQ ID NO:4. Most preferably, the cytoplasmic protein complex comprises a first recombinant fusion protein comprising SEQ ID NO:1 or SEQ ID NO:2 and a second recombinant fusion protein comprising SEQ ID NO:3 or SEQ ID NO:4.

In cases where an inactive mutant is used that can be activated by a small compound, one preferred embodiment is a mutant kinase whereby the mutant kinase is a Src 388R/A mutation or a 391R/A mutation in the corresponding human Src protein (Accession number NP_938033, version NP_938033.1). Another embodiment is a mutant wherein the mutant kinase is a Tyk2 1027R/A mutant (numbering of SEQ ID NO:26). Preferably, the small compound is imidazole.

The kinase of the first recombinant fusion protein is fused to a first interaction polypeptide, as defined below. The fusion may occur at the amino-terminal end, at the carboxy-terminal end, or internal in the recombinant polypeptide. In cases of kinases consisting of two domains, the first interaction polypeptide may even occur as an internal fusion in between the two domains. The domain comprising the reporter phosphorylation site of the second recombinant fusion protein is fused to a second interaction polypeptide, as defined below. The fusion may occur at the amino-terminal end, at the carboxy-terminal end, or internal in the recombinant polypeptide. The first recombinant fusion protein may not comprise a reporter phosphorylation site. The first recombinant fusion protein, the second recombinant fusion protein or both may further comprise other sequences such as, but not limited to, a localization signal to direct the cytoplasmic complex to a specific organelle into the cytoplasm of the cell, or to the nucleus.

Preferably, the cytoplasmic protein complex is phosphorylated at the reporter phosphorylation site; even more preferably, the phosphorylation is carried out by the kinase.

In an embodiment, the protein complex according to the invention is assembled by direct interaction of the first interaction polypeptide with the second interaction polypeptide. The first and the second interaction protein may be identical, in cases where homodimerization is studied. However, the protein complex is not limited to two compounds, and the interaction may be mediated by one or more other compounds. As a non-limiting example, the first interaction polypeptide may bind a protein that is bound on its turn by the second interaction polypeptide. Instead of a protein, the bridging compound can be one or more molecules of non-proteinous nature, or mixtures of small compounds and proteins. Another non-limiting example is a protein complex, wherein a small compound binding to the first, as well as to the second, interaction polypeptide.

The complex formation may be dependent upon modification of the first and/or second interaction polypeptide. Modification can be, but is not limited to, presence or absence of phosphorylation, acetylation, acylation, methylation, ubiquitinilation or glycosylation, or occurrence of proteolytic cleavage or not, or a combination thereof. Preferably, modification is carried out by a modifying enzyme (as defined below). Preferably, the modifying enzyme is fused to the first and/or second recombinant fusion protein. In case of phosphorylation, the modifying enzyme may be the constitutive kinase fused to the first interaction polypeptide, as described above.

Another aspect hereof is a method for detecting compound-compound interaction comprising (a) fusing a cytoplasmic kinase to a first interaction polypeptide resulting in a first recombinant fusion protein (without reporter phosphorylation site) according to the invention; (b) fusing a domain comprising a reporter phosphorylation site to a second interaction polypeptide, resulting in a second recombinant fusion protein according to the invention; (c) expressing both recombinant fusion proteins in a cell; and (d) identifying and/or selecting those cells in which the reporter phosphorylation site is phosphorylated.

Preferably, the kinase is a mutant kinase, even more preferably, a mutant tyrosine kinase. In one embodiment, the mutant kinase is a constitutive kinase; even more preferably, the kinase is a constitutive mutant derived from a Tyk2 kinase, preferably a deletion mutant or a constitutive V678F mutant. Alternatively, a constitutive Jak2 deletion mutant or a constitutive Src kinase deletion mutant can be used. In another embodiment, kinase is an inactive mutant kinase that is activated by addition of an exogenous small compound. Such mutant kinase is known to the person skilled in the art and has been described, as a non-limiting example, by Qiao et al. (2006).

In one embodiment, the first recombinant fusion protein comprises SEQ ID NO:1. In another embodiment, the first recombinant fusion protein comprises SEQ ID NO:2. In still another embodiment, the first recombinant protein comprises a sequence selected from the group consisting of SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, and SEQ ID NO:27. In one preferred embodiment, the second recombinant fusion protein comprises SEQ ID NO:3. In another preferred embodiment, the second recombinant fusion protein comprises SEQ ID NO:4. Most preferably, the cytoplasmic protein complex comprises a first recombinant fusion protein comprising SED ID NO:1 or SEQ ID NO:2 and a second recombinant fusion protein comprising SEQ ID NO:3 or SEQ ID NO:4.

The gene encoding the first and/or the second recombinant fusion protein may be placed downstream and is either a constitutive or an inducible promoter. The latter construction may have some advantages in cases where there is a competition for the binding site between interaction polypeptide and endogenous polypeptides. Induction of the recombinant fusion protein comprising the first interaction polypeptide in the presence of the second interaction polypeptides may facilitate the binding and avoid saturation of the binding sites with endogenous polypeptides. The cell may be any cell including, but not limited to, bacterial cells, fungal cells, yeast cells, insect cells and mammalian cells. Preferably, the cell is a eukaryotic cell; even more preferably, the cell is a mammalian cell. In cases where homodimerization is studied, the first and the second interaction protein may be identical. In this case, the same protein is fused to the kinase at one hand, and to a domain comprising the reporter phosphorylation site at the other hand.

The first recombinant fusion protein and the second recombinant fusion protein may be situated on one or on separated vectors. The vector used for transformation and expression of the first and/or second recombinant polypeptide can be any vector known to the person skilled in the art including, but not limited to, episomal vectors, integrative vectors and viral vectors.

In an embodiment, a eukaryotic cell carrying a gene encoding a first recombinant fusion protein is transformed or transfected with a vector library encoding second recombinant fusion proteins hereof. Interaction of the first interaction polypeptide with the second interaction polypeptide will result in phosphorylation of the reporter phosphorylation site and can be detected by the use of a reporter system. One specific embodiment of the method to detect compound-compound binding is a method whereby the compounds are proteins and binding is a protein-protein interaction. Another specific embodiment is a method to detect protein-protein interaction, whereby the interaction is modification state dependent, and modification is carried out by a modifying enzyme. Still another specific embodiment is a method to detect compound-compound binding, whereby binding is mediated by three or more partners. In this case, one or more partners may not be, or not completely be, of proteinous nature. It is clear for a person skilled in the art that the first interaction polypeptide may, as a non-limiting example, bind to a small molecule. On the other hand, the second interaction polypeptide may also bind to the small molecule, so that the first and the second are linked together by the small molecule. The small molecule may be present in the host cell, as a compound produced by the cell itself, or as a compound that is taken up from the medium.

Preferably, the method to detect compound-compound binding comprises the construction of a eukaryotic cell comprising a first recombinant fusion protein hereof, followed by transformation or transfection of the cell by a library of vectors encoding second recombinant fusion proteins according to the invention. The compound-compound binding is detected by the phosphorylation of the reporter phosphorylation site. Detection of the phosphorylation of the complex can be realized by isolation of the complex and analyzing the phosphorylation (e.g., by using radioactive labeled phosphor). Preferably, phosphorylation of the reporter phosphorylation site is resulting in the induction of a reporter system. A reporter system can be any system that allows the detection and/or the selection of the cells carrying a phosphorylated cytoplasmic protein complex according to the invention. It is clear for the person skilled in the art that several reporter systems can be used. As a non-limiting example, phosphorylation may lead to the induction of a signaling pathway, and a luciferase gene; an antibiotic resistance gene or a cell surface marker gene can be placed under control of a promoter that is induced by the signaling pathway. Alternatively, reporter systems may be used that are based on the change in characteristics of compounds of the signaling pathway, when the pathway is active, such as the phosphorylation and/or dimerization of such compounds. Still another possibility is using antibodies and/or other binding polypeptides which specifically recognize the phosphorylated reporter phosphorylation site.

One embodiment is a reporter phosphorylation site that is part of a Signal Transducer and Activator of Transcription (STAT) binding site, most preferentially, part of a STAT1 and/or STAT3 binding site. In this case, the reporter gene can be placed under the control of a STAT-inducible promoter, such as, but not limited to, the Pancreatitis Associated Protein 1 (rPAP) promoter. Alternatively, as phosphorylation of the reporter phosphorylation site will result in binding of a STAT polypeptide to the phosphorylated reporter phosphorylation site, followed by phosphorylation of the STAT polypeptide and subsequent dimerization of two phosphorylated STAT molecules, the dimerization itself can be used as reporter signal. Still another alternative reporter system consists of a protein complementation assay, wherein one part of the protein is incorporated in or associated with the cytoplasmic protein complex according to the invention, and the second part of the protein is recruited to the phosphorylated reporter phosphorylation site, leading to a detectable activity of the reconstituted protein.

Still another aspect hereof is a cell comprising a cytoplasmic protein complex according to the invention. This cell may be any cell including, but not limited to, bacterial cells, fungal cells, yeast cells, insect cells and mammalian cells. Preferably, the cell is a eukaryotic cell; even more preferably, the cell is a mammalian cell. Preferably, the reporter phosphorylation site in the cytoplasmic protein complex is phosphorylated. Preferably, the cell also comprises a reporter system, allowing the detection of the phosphorylation of the reporter phosphorylation site of the cytoplasmic protein complex.

Still another aspect hereof is a method for detecting compounds disrupting a polypeptide-polypeptide interaction, the method comprising (a) growing a cell comprising a protein complex according to the invention, wherein the cytoplasmic protein complex comprises the polypeptide-polypeptide interaction that one wants to disrupt, in absence and in presence of at least one compound; (b) comparing the phosphorylation of the reporter phosphorylation site in the second recombinant fusion protein of the cytoplasmic protein complex of cells, grown in presence or absence of the compound; and (c) identifying and/or selecting those cells wherein the reporter phosphorylation site is not phosphorylated. It is clear for the person skilled in the art that the polypeptide-polypeptide interaction as mentioned above may be an indirect interaction by three of more partners, whereby even one or more of the partners may not be of proteinous nature.

Definitions

The following definitions are set forth to illustrate and define the meaning and scope of various terms used to describe the invention herein.

“Protein” as used herein means a chain composed of amino acids, independent of the length. The terms “protein” and “polypeptide” are interchangeable. The protein can be modified by modifications such as, but not limited to, phosphorylation, glycosylation, ubiquitinilation and acetylation.

“Domain” as used herein is a part of a polypeptide, wherein the part may carry a specific function, such as, but not limited to, an enzymatic center or a phosphorylation site.

“Protein complex” as used herein means a structure that comprises at least two, non-covalently linked, protein molecules. Protein complexes can consist of more than two proteins and include other molecules that are not proteins. Some non-limiting examples of such molecules are metal ions, ATP, or carbohydrate molecules.

“Cytoplasmic protein complex” means that the protein complex as described above moves freely in the cytoplasm of the cell and is not linked to the cell membrane. However, the cytoplasmic protein complex may be directed to the nucleus or cytoplasmic organelles, using localization signal sequences.

A “kinase” as used herein is a polypeptide that can transfer a phosphate group to an amino acid of the same or another polypeptide. Preferably, the amino acid is a serine, a threonine or a tyrosine. Even more preferably, the amino acid is embedded in a phosphorylation site.

A “phosphorylation site” as used herein is a pattern of several amino acids, preferably comprising a serine, threonine or a tyrosine, and determining the amino acid that will be phosphorylated by the kinase. Most kinases can occur in an inactive state and/or in an active state, wherein the reporter phosphorylation site is only phosphorylated in the active state of the kinase. Kinases can be switched from the inactive form to the active form by phosphorylation, or by other modifications, such as proteolysis, or by mutation. The phosphorylation can be autophosphorylation, cross-phosphorylation (by a protein complex of identical kinases) or by action of another kinase.

A “cytoplasmic kinase” is a kinase that is freely moving in the cytoplasm, and not linked or recruited to the cellular membrane. The cytoplasmic kinase according to the invention may be derived from a membrane-linked kinase, by deleting the membrane anchoring or membrane recruitment domain, without loss of the enzymatic kinase activity.

A “mutant kinase” is a kinase of which the sequence differs from the sequence that occurs normally in nature, by the replacement, deletion or insertion of one or more amino acids. Homologues, orthologues and paralogues are not considered as mutants, as they are present as a normal form in nature.

“Constitutive” as used herein means that the kinase is continuously in the active state, normally as a consequence of a mutation, or by proteolytic cleavage removing an inhibitor. Constitutive kinases are known to the person skilled in the art and comprise, but are not limited to, truncated forms of Tyk2, truncated forms of Src kinase and point mutations such as Tyk2 (V678F), Jak1 (V658F) and Jak2(V617F).

An “inactive kinase mutant” means that that the mutant form shows a kinase activity that is significantly lower than the original non-mutated form. Preferably, the remaining activity is lower than 50% of the original activity, more preferably lower than 20%, even more preferably lower than 10%, and most preferably lower than 5% of the original activity.

“Activated by the addition of an exogenous small compound” as used herein means that the activity of the inactive kinase is partly or totally restored by addition of a small compound to the cells, whereby the small compound, exogenous to the cell, is taken up by the cell and activates the kinase as an intracellular exogenous compound. “Activated by the addition of an exogenous small compound” is used to make a distinction with ligand-receptor-induced activation, where a ligand is binding to the extracellular part of a receptor, and induces activation of the kinase.

“Exogenous” as used herein means that the compound is normally not present in the cell.

“Reporter phosphorylation site” is the site that is phosphorylated in the protein complex upon interaction of the first and the second interaction polypeptide; it is distinct from a possible phosphorylation site in the kinase domain that is autophosphorylated in the constitutive kinase.

“First interaction polypeptide” as used herein is a polypeptide of which one wants to study the interaction with one or more compounds. The first interaction polypeptide is normally referred to as a “bait” in the two-hybrid terminology.

“Second interaction polypeptide” as used herein is a polypeptide that is presented to study its interaction with the first interaction polypeptide. The second interaction polypeptide is normally referred to as a “prey” in the two-hybrid terminology. It is clear for the person skilled in the art that the first and the second interaction polypeptide are interchangeable in the invention, in this respect that either a “bait” or “prey” may be fused to constitutive kinase hereof. Indeed, the resulting protein complex will have an identical overall composition, composed of the four essential elements (first interaction polypeptide, second interaction polypeptide, constitutive kinase and reporter phosphorylation site), and independent whether the first interaction polypeptide is fused to the constitutive kinase or the reporter phosphorylation site (wherein the second interaction polypeptide is then fused to the reporter phosphorylation site and the constitutive kinase, respectively), the interaction of the first with the second interacting polypeptide will lead to the formation of a cytoplasmic protein complex according to the invention, and will result in the phosphorylation of the reporter phosphorylation site. In one preferred embodiment, the first and the second interaction proteins are identical to study homodimerization or homomultimerization of a protein. In another preferred embodiment, the first and the second proteins are different, allowing the study of protein-protein interactions of heterodimers or heteromultimers.

“Modifying enzyme” as used herein means any enzymatic activity that results in a modification of the first and/or second interaction polypeptide. Such modification can be, as a non-limiting example, phosphorylation, acetylation, acylation, methylation, ubiquitinilation or glycosylation, occurrence of proteolytic cleavage, or a combination thereof. Preferably, the modification is phosphorylation. The modifying enzyme may be associated to or incorporated in the first and/or the second recombinant fusion protein of the cytoplasmic protein complex. It may be co-expressed, with the first and/or second interaction polypeptide, or it can be fused to one or both of the polypeptides. In case of phosphorylation, the constitutive kinase of the cytoplasmic protein complex according to the invention may carry out both the phosphorylation of the activation site as well as the phosphorylation of one or both of the interaction polypeptides.

“Compound” means any chemical or biological compound, including simple or complex organic or inorganic molecules, peptides, peptido-mimetics, proteins, antibodies, carbohydrates, nucleic acids or derivatives thereof.

“Interaction” means any interaction, be it direct or indirect. A direct interaction implies a contact between the interaction partners. An indirect interaction means any interaction whereby the interaction partners interact in a complex of more than two compounds. This interaction can be completely indirect, with the help of one or more bridging compounds, or partly indirect, where there is still a direct contact that is stabilized by the interaction of one or more compounds.

EXAMPLES Materials and Methods to the Invention Plasmids Used in the Examples

A first type of plasmids express chimeric proteins consisting of an HA-tagged C-terminal portion of human Tyk2 fused at its C-terminus to the first interacting polypeptide and are generated in the pMet7 vector, which contains a strong constitutive hybrid SRα promoter (Takebe et al., 1988). To generate the pMet7-HA-Tyk2(C)-RTp66 plasmid, the sequence encoding the C-terminal end of human Tyk2 comprising the kinase domain (starting from amino acids 589 and omitting the stop codon) was amplified by PCR on cDNA from HEK293 cells with primers MBU-O-6486 and MBU-O-6487. In addition to an HA coding sequence, the former primer contained an ApaI restriction enzyme recognition site, whereas the latter primer contained an EcoRI restriction enzyme recognition site. The PCR amplicon was digested with ApaI and EcoRI and ligated in the ApaI-EcoRI cut pMG2-RTp66 plasmid (a pMet7-derived plasmid encoding a Flag-gp130-RTp66 chimeric protein), which contained ApaI and EcoRI sites flanking the sequences encoding the Flag-gp130 fusion protein at the 5′ and 3′ end, respectively (Pattyn et al., 2008). pMet7-HA-Tyk2(C)-RTp51 was generated by exchanging the RTp51 insert from pMG2-RTp51 (Pattyn et al., 2008) with RTp66 from pMet7-HA-Tyk2(C)-RTp66 using the EcoRI and NotI sites that flank these inserts at the 5′ and 3′ end, respectively. Full length human LEDGF was PCR amplified on cDNA from HeLa cells using primers MBU-O-3879 and MBU-O-3880 and exchanged with the RTp66 insert of pMet7-HA-Tyk2(C)-RTp66 using EcoRI and NotI restriction sites to generate pMet7-HA-Tyk2(C)-LEDGF. Similarly, the full-length human MDM2 sequence was PCR amplified on an MDM2 entry clone from the hORFeome collection (Lamesch et al., 2007) with primers MBU-O-3912 and MBU-O-3913 and exchanged with EcoRI-NotI to produce pMet7-HA-Tyk2(C)-MDM2. The N-terminal region of human p53 (amino acids 2-72) was PCR amplified on a p53 entry clone from the hORFeome collection (Lamesch et al., 2007) with primers MBU-O-2277 and MBU-O-2273 and exchanged using EcoRI and NotI restriction enzymes, yielding pMet7-HA-Tyk2(C)-p53(N). The plasmid pMet7-HA-Jak2(C)-RTp66 was generated by PCR amplifying the sequence encoding the C-terminal end of mouse Jak2 (from amino acid 535 until the end of the protein, leaving the stop codon out) on pRK5-mJak2 (Silvennoinen et al., 1993) with primers MBU-O-6653 and MBU-O-6655. This fragment was exchanged with the sequence encoding Tyk2(C) from pMet7-HA-Tyk2(C)-RTp66 through a PacI-EcoRI restriction digest. Likewise, the coding sequence for the kinase domain of c-Src (amino acids 266-523) was PCR amplified on a c-Src entry clone from the hORFeome collection (Lamesch et al., 2007) with primers MBU-O-6656 and MBU-O-6657 and exchanged using PacI and EcoRI restriction enzymes, generating pMet7-HA-c-Src(K)-RTp66. The pMet7-HA-Tyk2(R1027A)-RTp66 plasmid was generated by first PCR amplifying the human Tyk2 sequence on HEK293 cDNA with primers MBU-O-7811 and MBU-O-7812 and exchanging the resulting amplicon for the Tyk2(C) sequence of pMet7-HA-Tyk2(C)-RTp66 using PacI and EcoRI restriction enzymes to yield pMet7-HA-Tyk2-RTp66. Next, the R1027A mutation was introduced by site-directed mutagenesis using primers MBU-O-7341 and MBU-O-7342.

The plasmids encoding the fusions with the second interacting polypeptide were of the type also used in MAPPIT, designated pMG1 and pMG2 (WO0190188, Eyckerman et al., 2001; Lemmens et al., 2003). These plasmids encode fusion proteins of the second interacting polypeptide coupled to a fragment of the human gp130 cytokine receptor chain, which contains multiple tyrosine residues that, upon phosphorylation, make up recruitment sites for STAT3. RTp66 and RTp51 containing plasmids pMG2-RTp66 and pMG2-RTp51 have been described elsewhere (Pattyn et al., 2008). The pMG1 plasmid encoding an unfused gp130 receptor fragment was obtained by cutting out the MAPPIT prey insert of a pMG1 vector using EcoRI and XhoI, blunting the vector backbone through Pfu DNA Polymerase and self-ligation. The pMG2-IN plasmid was constructed by PCR amplifying the sequence encoding HIV1 integrase on the pNL4-3 plasmid template (Adachi et al., 1986) using primers MBU-O-3813 and MBU-O-3814 and exchanging this sequence with the insert of a pMG2 MAPPIT vector using EcoRI and NotI restriction enzymes. Similarly, the coding region of human MDM2 was PCR amplified with primers MBU-O-3912 and MBU-O-3913 and exchanged with EcoRI-NotI to produce pMG2-MDM2. pMG1-p53 and pMG1-EFHA1 were generated by Gateway recombination mediated transfer of the full-length sequences of human p53 and EFHA1 from entry vectors of the hORFeome collection (Lamesch et al., 2007) into a Gateway-compatible version of the pMG1 vector as described earlier (Lievens et al., 2009).

The reporter plasmid pXP2d2-rPAPI-luciferase used in the examples contains the STAT3-dependent rPAPI (rat Pancreatitis-Associated Protein I) promoter driving a firefly luciferase reporter gene as described previously (WO0190188, Eyckerman et al., 2001).

Transfection Procedure

Transfections were carried out using a standard calcium phosphate method. HEK293-T cells were seeded in black tissue-culture treated 96-well plates at 10,000 cells/well in 100 μl culture medium (DMEM supplemented with 10% FCS). Twenty-four hours later, plasmid DNA mixes were prepared that contained plasmids encoding fusion proteins with the first and second interacting proteins and reporter plasmids. The DNA was supplemented with 10 μl 2.5M CaCl₂ and double distilled water to a final volume of 100 μl. This mixture was added dropwise to 100 μl 2×HeBS buffer (280 mM NaCl, 1.5 mM Na₂HPO₄, 50 mM Hepes; pH 7.05) while vigorously vortexing. After incubation at room temperature for 15 minutes to allow DNA precipitates to form, the solution was added to the cells at 10 μl/well. Cells were incubated at 37° C., 8% CO₂. Forty-eight hours after transfection, luciferase activity was measured using the Luciferase Assay System kit (Promega) on a TopCount luminometer (Perkin-Elmer). Each transfection was done in triplicate and the average of the luciferase activity readings was used in the calculations.

A Nutlin-3 (Sigma) stock solution of 20 mM in DMSO was diluted in culture medium and added to the cells 24 hours after transfection. Imidazole (Sigma) diluted in culture medium was added to the cells 24 hours after transfection.

Oligonucleotide SEQ ID primer code NO: Sequence (5′ > 3′) MBU-O-6486 8 CCCGGGCCCACCATGTATCCATATGATGTTCCAGATT ATGCTTTAATTAAAATCACCCAGCTGTCCCACTTGG MBU-O-6487 9 GGGGAATTCGCACACGCTGAACACTGAAGG MBU-O-3879 10 CGTACGAATTCGGGAGCTCGATGACTCGCGATTTCAA ACCTGGAG MBU-O-3880 11 GGTCATCTAGACCGCGGCCGCTCAGTTATCTAGTGTA GAATCCTTCAG MBU-O-3912 12 GCGGAATTCATGTGCAATACCAACATGTCTG MBU-O-3913 13 CGCGCGGCCGCCTAGGGGAAATAAGTTAGCAC MBU-O-2277 14 GCGAGAATTCGAGGAGCCGCAGTCAGATCC MBU-O-2273 15 CGCTGCGGCCGCTTAGCGGGGAGCAGCCTCTGGC MBU-O-6653 16 CCCGCGGCCGCTTTAATTAAAATGGTGTTTCACAAAA TCAG MBU-O-6655 17 GGGCTCGAGGAATTCCGCAGCTATACTGTCCCGG MBU-O-6656 18 CCCGCGGCCGCTTTAATTAAACCTCGGGAGTCGCTGC GGC MBU-O-6657 19 GGGCTCGAGGAATTCGAAGTAGTCCTCCAGGAAGG MBU-O-3813 20 CGTACGAATTCGGGAGCTCGTTTTTAGATGGAATAG MBU-O-3814 21 GGTCATCTAGACCGCGGCCGCTCAATCCTCATCCTGT CTAC MBU-O-7811 22 CATTTAATTAAACCTCTGCGCCACTGGGGG MBU-O-7812 23 CATGAATTCGCACACGCTGAACACTGAAGGG MBU-O-7341 24 CCGAGACCTAGCCGCGGCCAACGTGCTGC MBU-O-7342 25 GCAGCACGTTGGCCGCGGCTAGGTCTCGG

Example 1: Detection of the Interaction Between HIV1 Reverse Transcriptase (RT) Subunits

In order to determine the functionality of the assay, the interaction between HIV1 subunits that form homo- and heterodimers was tested by transfecting the following combinations of plasmids (100 ng of the Tyk2(C) fusion construct, 1 μg of the gp130 fusion construct and 50 ng of the luciferase reporter construct) according to the methods described above:

-   -   a) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp51+pXP2d2-rPAPI-luciferase     -   b) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase     -   c) pMet7-HA-Tyk2(C)-RTp51+pMG2-RTp51+pXP2d2-rPAPI-luciferase

Background signal for each Tyk2(C) fusion polypeptide was determined by transfecting the plasmid that encodes it together with a plasmid encoding an unfused gp130 fragment (pMG1) and the luciferase reporter plasmid (pMet7-HA-Tyk2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in a) and b); pMet7-HA-Tyk2(C)-RTp51+pMG1+pXP2d2-rPAPI-luciferase in c)). The fold induction for each tested interaction was calculated as the ratio of the luciferase activity measured relative to the luciferase activity measured for the corresponding background signal. The results (FIG. 2A) show clear signals for both RT homo- and heterodimers. The strongest signal was that for the p66-p51 heterodimer; lower, but still robust, signals were obtained for the p66 and p51 homodimers. This trend corresponds to the affinities measured for p66-p51, p66-p66 and p51-p51 interactions in in vitro interaction assays, which were reported to be 0.3 μM, 4 μM and 230 μM, respectively (Venezia et al., 2006). These data illustrate the high sensitivity of the method.

Example 2: Interaction Between Nuclear Proteins

To determine whether the method can detect interactions between nuclear proteins, we tested the interaction between HIV1 Integrase (IN) and human LEDGF and between human p53 and MDM2, proteins with a nuclear localization. Cells were transfected with the following combinations of plasmids (250 ng of the Tyk2(C) fusion construct, 500 ng of the gp130 fusion construct and 50 ng of the luciferase reporter construct) according to the methods described above:

-   -   a) pMet7-HA-Tyk2(C)-LEDGF+pMG2-IN+pXP2d2-rPAPI-luciferase     -   b) pMet7-HA-Tyk2(C)-MDM2+pMG1-p53+pXP2d2-rPAPI-luciferase

Background signal for each Tyk2(C) fusion polypeptide was determined by transfecting the plasmid that encodes it together with a plasmid encoding an unfused gp130 fragment (pMG1) and the luciferase reporter plasmid (pMet7-HA-Tyk2(C)-LEDGF+pMG1+pXP2d2-rPAPI-luciferase in a); pMet7-HA-Tyk2(C)-MDM2+pMG1+pXP2d2-rPAPI-luciferase in b)). The fold induction for each tested interaction was calculated as the ratio of the luciferase activity measured relative to the luciferase activity measured for the corresponding background signal. The result (FIG. 3) shows that these interactions give rise to strong signals, indicating that the method is able to detect interactions in the nucleus.

Example 3: Dose-Dependent Disruption of the Interaction Between Human p53 and MDM2 by Nutlin-3

To show that the method can detect modulation of protein-protein interactions by small molecules, we analyzed the interaction between p53 and MDM2, which has been reported to be disrupted by Nutlin-3, a member of the nutlin family of potential novel anti-cancer compounds (Vassilev et al., 2004). Cells were transfected with the following combinations of plasmids (100 ng of the Tyk2(C) fusion construct, 1000 ng of the gp130 fusion construct and 50 ng of the luciferase reporter construct) according to the methods described above:

-   -   a) pMet7-HA-Tyk2(C)-p53(N)+pMG1+pXP2d2-rPAPI-luciferase     -   b) pMet7-HA-Tyk2(C)-p53(N)+pMG1-EFHA1+pXP2d2-rPAPI-luciferase     -   c) pMet7-HA-Tyk2(C)-p53(N)+pMG2-MDM2+pXP2 d2-rPAPI-luciferase

After transfection, cells were treated with 0-14-41-123-370-1111-3333 nM Nutlin-3, final concentration. The results shown in FIG. 4 show a dose-dependent decrease of the signal for the interaction between p53 (N-terminal region containing the MDM2 binding domain) and MDM2 upon treatment with Nutlin-3, whereas the signal corresponding to the interaction with EFHA1 (which binds to Tyk2(C) itself) is unaffected by this treatment. The signal from a control transfection combining pMet7-HA-Tyk2(C)-p53(N) and the pMG1 plasmid encoding an unfused gp130 receptor fragment is low and also unaffected by Nutlin-3 treatment. These data indicate that the method can detect dynamic changes in protein complexes dependent on addition of exogenous components.

Example 4: Incorporation of Other Kinase Domains in the Method

To support the fact that the method is not limited to the use of the kinase domain derived from Tyk2, we tested fusion proteins comprising the kinase domains of Jak2 or c-Src. Similarly to Tyk2, Jak2 belongs to the Jak family of receptor-associated tyrosine kinases. c-Src belongs to the Src non-receptor tyrosine kinase family. Cells were transfected with the following combinations of plasmids (250 ng of the kinase fusion construct, 1000 ng of the gp130 fusion construct and 50 ng of the luciferase reporter construct) according to the methods described above:

-   -   a) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase     -   b) pMet7-HA-Jak2(C)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase     -   c) pMet7-HA-c-Src(K)-RTp66+pMG2-RTp66+pXP2d2-rPAPI-luciferase

Luciferase activity is shown as fold induction relative to the luciferase activity measured in cells transfected with the same RTp66-kinase domain fusion, an unfused gp130 fragment and the luciferase reporter plasmid (pMet7-HA-Tyk2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in a); pMet7-HA-Jak2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in b); pMet7-HA-c-Src(K)-RTp66+pMG1+pXP2d2-rPAPI-luciferase in c)). The fold induction for each tested interaction was calculated as the ratio of the luciferase activity measured relative to the luciferase activity measured for the corresponding background signal. The result (FIG. 5) indicates that the interaction between RTp66 subunits can also be detected using fusion proteins with kinase domains of other kinases than that of Tyk2.

Example 5: Inducible Version of the Method

An inducible version of the method was devised to add an additional level of control and to allow temporal separation of the protein interaction and signal generation events. The latter might be important in cases where modification of the interaction polypeptides by the kinase activity prohibits interaction. A mutant Src tyrosine kinase has been described, the activity of which is made inducible by the small molecule imidazole by an arginine to alanine mutation in the catalytic center of the enzyme (Qiao et al., 2006). The corresponding point mutation of this conserved catalytic arginine (R1027) was generated in Tyk2, fused to RTp66 and tested in an imidazole-inducible version of the assay. Cells were transfected with the following combinations of plasmids (250 ng of the kinase fusion construct, 250 ng of the gp130 fusion construct and 50 ng of the luciferase reporter construct) according to the methods described above:

-   -   a) pMet7-HA-Tyk2(C)-RTp66+pMG1+pXP2d2-rPAPI-luciferase     -   b) pMet7-HA-Tyk2(C)-RTp66+pMG2-RTp51+pXP2d2-rPAPI-luciferase     -   c) pMet7-HA-Tyk2(R1027A)-RTp66+pMG1+pXP2d2-rPAPI-luciferase     -   d)         pMet7-HA-Tyk2(R1027A)-RTp66+pMG2-RTp51+pXP2d2-rPAPI-luciferase

After transfection, cells were treated with 0.5-50 mM imidazole, final concentration. The results are represented in FIG. 5, showing an imidazole-inducible signal specifically in cells expressing a fusion containing the Tyk2 arginine to alanine mutation. Maximal imidazole-dependent induction of the signal, indicative of the interaction between HIV RTp66 and RTp51, was compared 36-fold to untreated cells. Cells transfected with the Tyk2(C) fusion described earlier did not exhibit any imidazole regulation. Background signal observed in cells transfected with combinations of the Tyk2 fusion construct and a plasmid encoding an unfused gp130 fragment (pMG1) similarly was unaffected by imidazole treatment. These results indicate that it is possible to control the interaction assay by applying a chemically inducible kinase.

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The invention claimed is:
 1. A cytoplasmic protein complex comprising: a first recombinant fusion protein comprising a monomeric mutant tyrosine kinase fused to a first interaction polypeptide, wherein the monomeric mutant tyrosine kinase is continuously in the active state; a second recombinant fusion protein comprising a domain fused to a second interaction polypeptide, wherein the domain comprises a reporter phosphorylation site distinct from any phosphorylation site in the monomeric mutant tyrosine kinase; wherein the cytoplasmic protein complex is assembled by interaction between the first and second interaction polypeptides; wherein the cytoplasmic protein complex is phosphorylatable at the reporter phosphorylation site by the monomeric mutant tyrosine kinase in the cytoplasmic protein complex; and wherein the monomeric mutant tyrosine kinase comprises a sequence of at least 95% identity to SEQ ID NO: 1 and wherein said sequence has tyrosine kinase activity.
 2. The cytoplasmic protein complex according to claim 1, wherein the first and second interaction polypeptides are not identical.
 3. The cytoplasmic protein complex according to claim 1, wherein the monomeric mutant tyrosine kinase comprises a V678F mutation.
 4. A cytoplasmic protein complex comprising: a first recombinant fusion protein comprising a monomeric mutant tyrosine kinase fused to a first interaction polypeptide, wherein the monomeric mutant tyrosine kinase is continuously in the active state or is inactive and activated by addition of an exogenous small compound; a second recombinant fusion protein comprising a domain fused to a second interaction polypeptide, wherein the domain comprises a reporter phosphorylation site distinct from any phosphorylation site in the monomeric mutant tyrosine kinase; wherein the cytoplasmic protein complex is assembled by interaction between the first and second interaction polypeptides; wherein the cytoplasmic protein complex is phosphorylatable at the reporter phosphorylation site by the monomeric mutant tyrosine kinase in the cytoplasmic protein complex; and wherein the first recombinant fusion protein comprises SEQ ID NO:
 1. 5. The cytoplasmic protein complex according to claim 1, wherein the second recombinant fusion protein comprises SEQ ID NO:3.
 6. A method of using the cytoplasmic protein complex of claim 1 to detect a compound-compound interaction, the method comprising: expressing both the first recombinant and second recombinant fusion proteins of the cytoplasmic protein complex in a cell; and identifying and/or selecting those cells in which the reporter phosphorylation site is phosphorylated.
 7. The method according to claim 6, wherein the cell is a eukaryotic cell.
 8. The method according to claim 7, wherein the eukaryotic cell is a mammalian cell.
 9. The method according to claim 6, wherein the kinase comprises SEQ ID NO:
 1. 10. The method according to claim 6, wherein the second recombinant fusion protein comprises SEQ ID NO:3.
 11. An isolated cell comprising the cytoplasmic protein complex of claim
 1. 12. The isolated cell of claim 11, wherein the cell is a eukaryotic cell.
 13. The isolated cell of claim 12, wherein the eukaryotic cell is a mammalian cell.
 14. A method of using the isolated cell of claim 11 for detecting a compound that disrupts a polypeptide-polypeptide interaction, the method comprising: growing the isolated cell, wherein the cytoplasmic protein complex thereof comprises the polypeptide-polypeptide interaction to be disrupted, in the absence of and in the presence of at least one compound; comparing the phosphorylation of the reporter phosphorylation site in the second recombinant fusion protein of the cytoplasmic protein complex of cells, grown in the presence of and in the absence of the at least one compound; and identifying and/or selecting cells wherein the reporter phosphorylation site is not phosphorylated.
 15. The cytoplasmic protein complex of claim 1, wherein the reporter phosphorylation site is part of a Signal Transducer and Activator of Transcription (STAT) binding site.
 16. The cytoplasmic protein complex of claim 1, wherein the monomeric mutant tyrosine kinase is a Tyk2 mutant.
 17. A cytoplasmic protein complex comprising: a first recombinant fusion protein comprising a monomeric mutant Tyk2 kinase fused to a first interaction polypeptide; a second recombinant fusion protein comprising a gp130 domain comprising at least one phosphorylation site that can be phosphorylated by the monomeric mutant Tyk2 kinase, wherein the gp130 domain is fused to a second interaction polypeptide; wherein the first interaction polypeptide is bound to the second interaction polypeptide; and wherein a wild-type form of the mutant Tyk2 kinase can phosphorylate a wild-type gp130 domain comprising the phosphorylation site for Tyk2. 